Encapsulating toxic Rhodamine 6G dye, and Cr (VI) metal ions from liquid phase using AlPO4-5 molecular sieves. Preparation, characterization, and adsorption parameters

Abstract

Water contamination through textile dyes, metal depositions, and industrial wastes such as coal combustion residues especially coal fly ash (CFA) putting an enormous strain on dwindling planets water resources. Several characterization techniques, such as XRF, XRD, BET, SEM, and FT-IR, were performed to test the inherent properties of raw CFA and the product zeolite (AlPO4-5). The fusion ratio (CFA: NaOH) substantially enhanced the specific surface area of the zeolite (219.3 m2/g) compared to raw CFA (6.27 m2/g). The hydrothermal treatment greatly altered the molar ratio of Si/Al from 3.18 (CFA) to 2.11 (AlPO4-5) suggesting higher crystallinity of the product. Further, AlPO4-5 molecular sieve was tested its ability in separation of cationic Rhodamine 6G dye (R 6G), and Cr (VI) from liquid phase via batch mode adsorption process. The results indicated that the optimal adsorption conditions were (pH = 7 for R 6G; pH = 2 for Cr (VI); temperature 303 K for both adsorbates; dosage (0.05 g/30 mL–R 6G; 0.1 g/30 mL–Cr(VI); and residence time; 120 min–R 6G, 180 min–Cr (VI) respectively. Further, the Langmuir isotherm model suits well reporting maximum adsorption capacities (qm) as 208.61 (mg/g)–R 6G; 97.32 (mg/g)–Cr (VI)), and best fits to the pseudo-second-order kinetic models. The thermodynamic aspects exhibit the spontaneous and endothermic nature of R 6G dye, and Cr (VI) metal sorption onto AlPO4-5 zeolite. From the desorption studies, alkaline eluent 0.1 M NaOH showed better performance in recovering 89.4%, 91% of adsorbent from R 6G dye, and Cr (VI) solutions respectively. Lastly, the reusability studies confirmed that the adsorbent (AlPO4-5) had the potential up to three cycles (R 6G), and it is confined to two cycles in case of Cr (VI) metal solution.